1. Field of the Invention
The invention relates to a semiconductor acceleration sensor which is used for a portable terminal equipment, a toy, an automobile, an aircraft and the like to detect an acceleration.
2. Description of the Related Art
The following will discuss the configuration of a conventional three-axis acceleration sensor using piezo resistors. FIG. 15 is an exploded perspective view showing the acceleration sensor. In such an acceleration sensor, an acceleration sensor element 200 is fixed on a case 2 with adhesive and a lid 3 is fixed on the protection case 2 with adhesive. External terminals 7 of the acceleration sensor element 200 and terminals 5 of the protection case 2 are connected via metal wires 4. The output of the acceleration sensor element 200 is drawn from external terminals 6 of the protection case 2 to the outside. In this specification, the acceleration sensor element 200 will be referred to as an acceleration sensor.
FIG. 16 is a plan view showing the acceleration sensor 200. In FIG. 16, wires and external terminals on a support frame are omitted to facilitate the understanding of the configuration of the piezo resistors. The acceleration sensor 200 is constituted of a mass portion 13 made up of a thick portion of a silicon single crystal substrate, a support frame 11 surrounding the mass portion 13, two orthogonal pairs of beam-like flexible arms 21 and 21′ and 22 and 22′ which are made up of thin portions of the silicon single crystal substrate and connect the mass portion 13 and the support frame 11, and a plurality of piezo resistors 51, 51′, 52, 52′, 61, 61′, 62, 62′, 71, 71′, 72 and 72′ on axes corresponding to two directions (X and Y) of the top surface of the flexible arm and a direction (Z) perpendicular to the top surface of the flexible arm. Further, the flexible arms 21, 21′, 22 and 22′ are shaped like beams by through holes 150 on the thin portions, and thus the flexible arms are easily deformed and are suitable for higher sensitivity.
In the conventional acceleration sensor, the piezo resistors are provided such that the ends of the X-axis piezo resistors 51 and 51′ and the Z-axis piezo resistors 71 and 71′ are aligned with the boundary of the flexible arm 21 with the support frame 11 and the boundary of the flexible arm 11 with the mass portion 13, so that the maximum sensor output can be obtained.
It is generally known that when the piezo resistors are arranged as shown in FIG. 16, a relationship in the graph of FIG. 17 is formed between the sensitivities of the X axis and the Z axis (at an acceleration of 1 G, an output relative to a driving voltage of 1 V). When the thickness of the mass portion is changed, the sensitivity of the X-axis changes like a quadratic function. The sensitivity of the Z axis changes like a linear function. Thus, the X axis and the Z axis are different in sensitivity. To eliminate an output difference, the thickness of the mass portion and the sensitivity of the piezo resistors are changed or the arrangement of the piezo resistors is changed.
In order to eliminate an output difference between the X axis and the Z axis, it is advisable to set the thickness of the mass portion at about 800 μm which enables the X axis and the Z axis to have an equal sensitivity. However, the thickness of Si single crystal substrates used for semiconductors and the like is mainly set at 625 μm and 525 μm, and thus Si single crystal substrates of about 800 μm have to be specially ordered, resulting in irregular delivery times as well as high cost. Hence, it is not preferable to adjust output by using the thickness of the mass portion.
The piezo resistors are formed by implanting impurity elements such as boron into a silicon single crystal substrate. The sensitivity of the piezo resistor can be varied by changing the concentration of impurity elements. At least several impurity implanting steps are necessary to change the concentration of impurity elements, resulting in higher manufacturing cost and lower equipment capacity. Thus, this method is not preferable.
Further, Japanese Patent Laid-Open No. 2003-279592 and Japanese Patent Laid-Open No. 2003-294781 propose the following technique: in order to eliminate an output difference among the X, Y, and Z axes, the output of the Z axis is reduced to those of the X and Y axes by changing the arrangement of piezo resistors. When the sensitivity of the Z axis is reduced to those of the X and Y axes having low outputs, the sensitivity of the Z axis is sacrificed. Moreover, the conventional acceleration sensor is configured such that the output of the Z axis is larger than that of the X axis, resulting in a large output difference between the axes. In the case of a large output difference between the axes, an amplifier with a different output amplification factor has to be prepared for each of the axes.